Automated inlet steam supply valve controls for a steam turbine powered chiller unit

ABSTRACT

A control system and method are provided for the controlling of steam supplies used by a steam turbine driven chiller unit. The steam turbine can receive steam from a high pressure steam source and/or a low pressure steam source depending on the operating mode of the steam turbine. The high pressure steam is used for operating at the steam turbine at rated speed and to provide the breakaway torque when starting the steam turbine. The low pressure steam is used for extending idling of the steam turbine that enables the steam turbine to transition more quickly to rated speed when desired.

BACKGROUND OF THE INVENTION

The present invention relates generally to a control system for achiller unit, and more specifically, to a control system for a steamturbine powered chiller unit that can control inlet steam supply valvesfor a steam turbine receiving steam from two different steam supplies.

While most heating, ventilation and air conditioning (HVAC),refrigeration, or chiller systems use electric motors to power thecorresponding compressor(s) in the chiller system, some chiller systemshave used a steam turbine to power the compressor. These previous steamturbine powered chiller systems were supplied with only a high pressuresteam supply required for normal full load operation and had a PLC basedpanel for use with the steam turbine drive. The panel logic controlledonly the remote speed set point of the electronic governor supplied bythe turbine manufacture. This stand-alone speed control prevented thecustomer from safely taking advantage of an available low pressure steamsupply during an extended idle period for the chiller system because itwas not possible to add to the PLC the adaptive tuning required tohandle changes in the motive force when switching between the high andlow pressure steam supplies.

Furthermore, the use of high pressure steam for extended idling wouldrequire sufficient cooling water flow through the steam condenser to bemaintained to prevent the steam condenser from overheating. Thus,instead of attempting to maintain the sufficient cooling water flow, thechiller system was completely stopped to prevent overheating and topermit the turbine casing to cool down before the next restart. Thestopping of the chiller system then resulted in the operator having toperform an extensive manual start up procedure and slow roll warm upbefore the turbine could be operated at rated speed again.

Therefore, what is needed is automated inlet steam supply valves for asteam turbine powered chiller unit and a corresponding control systemthat can control the providing of both low pressure steam and highpressure steam to the steam turbine with the inlet steam supply valves.

SUMMARY OF THE INVENTION

One embodiment of the present invention is directed to a method ofstarting a steam turbine driven chiller system having a high pressuresteam supply and a low pressure steam supply. The method includes thesteps of executing a starting sequence for the steam turbine, initiatinga slow roll of the steam turbine using the high pressure steam supply,transitioning from the high pressure steam supply to the low pressuresteam supply, and slow rolling the steam turbine at a predetermined slowroll speed using the low pressure steam supply.

Another embodiment of the present invention is directed to a method ofinitiating an idling mode in a steam turbine driven chiller systemhaving a high pressure steam supply and a low pressure steam supply. Themethod includes the steps of executing a transition sequence for thesteam turbine, initiating an unload cycle for the chiller system,transitioning from the high pressure steam supply to the low pressuresteam supply, and slow rolling the steam turbine at a predeterminedidling speed using the low pressure steam supply. The steam turbineoperates at a rated speed using the high pressure steam supply prior tothe transition sequence. The predetermined idling speed is less than therated speed.

Still another embodiment of the present invention is directed to achiller system having a steam system including a high pressure steamsupply, a low pressure steam supply, a steam turbine and a steamcondenser connected in a steam loop. The chiller system also has arefrigerant system including a compressor, a refrigerant condenser, andan evaporator connected in a refrigerant loop. The compressor is drivenby the steam turbine. The chiller system further has a control panel tocontrol operation of both the steam system and the refrigerant system.The control panel includes a control system to operate the steam systemin an idling mode using the low pressure steam supply. The idling modeoperation results in the steam turbine operating at a predetermined slowroll speed and no substantial output capacity from the refrigerantsystem.

One advantage of the present invention is that the starting mode and thereturn to a standby idling mode of the steam turbine can be controlledremotely by a control system.

Another advantage of the present invention is that a reduced coolingwater flow is required for the steam condenser during an extended idlingperiod of the steam turbine.

Other features and advantages of the present invention will be apparentfrom the following more detailed description of the preferredembodiment, taken in conjunction with the accompanying drawings whichillustrate, by way of example, the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a side view of a chiller unit of the present invention.

FIG. 2 is a top view of the chiller unit of FIG. 1.

FIG. 3 is a schematic representation of the chiller unit of FIG. 1.

FIG. 4 is a schematic representation of the control system of thechiller unit of FIG. 1.

FIG. 5 is a flowchart of an embodiment of a start-up process for thepresent invention.

FIGS. 6 and 7 are a flowchart of an embodiment of a ramp-up to ratedspeed process for the present invention.

FIGS. 8 and 9 are a flowchart of an embodiment of a return to idlingspeed process for the present invention.

Wherever possible, the same reference numbers will be used throughoutthe drawings to refer to the same or like parts.

DETAILED DESCRIPTION OF THE INVENTION

A general system to which the invention is applied is illustrated, bymeans of example, in FIGS. 1-3. As shown, the HVAC, refrigeration, orchiller system 10 includes a compressor 12, a steam turbine 14, arefrigerant condenser 16, a water chiller or evaporator 18, a steamcondenser 20, an expansion device 22 and a control panel or controller90. The operation of the control panel 90 will be discussed in greaterdetail below. The chiller system 10 further includes a compressorlubrication system (not shown) and a turbine lubrication system (notshown). The conventional liquid chiller system 10 includes many otherfeatures that are not shown in FIGS. 1-3. These features have beenpurposely omitted to simplify the drawing for ease of illustration.

In one embodiment, a “structural frame” permits the stacking or verticalarrangement of major components of the chiller system 10 to provide aprepackaged unit that occupies less floor space with a smaller footprintthan a field fabricated unit where the components are arrangedhorizontally. The structural frame can include a turbine baseplate 26, asteam condenser baseplate 27, a plurality of frame members 28, and tubeend sheets 29. Tube end sheets 29 can provide both the internal supportand refrigerant/water separation for the ends of heat exchange tubes(not shown) within refrigerant condenser 16 and evaporator 18. Framemembers 28 are preselected structural components and materials, such asplate steel and tubular supports, that can support the correspondingcomponents of the chiller system 10. The mounting between compressor 12and turbine baseplate 26 is preferably a conventional D-flange couplingdevice that rigidly interconnects the housing of the compressor 12 withthe turbine baseplate 26. In addition, the D-flange coupling device canafford a predictable degree of shaft alignment for the compressor 12 andthe steam turbine 14.

The structural frame can incorporate a steam turbine 14 in combinationwith a refrigerant condenser 16, evaporator 18 and compressor 12 into apre-packaged unit for installation. The steam condenser 20 and steamcondenser baseplate 27 can then be manufactured as a separate unit fromthe pre-packaged unit and include all necessary interconnections forconnection to the pre-packaged unit. The steam condenser 20 and steamcondenser baseplate 27 can be field installed above the refrigerantcondenser 16 during installation of chiller system 10. Finally, inanother embodiment of the present invention, the main components of thechiller system 10 can be field installed into any suitable or desirablepositions.

In the chiller system 10, the compressor 12 compresses a refrigerantvapor and delivers it to the refrigerant condenser 16. The compressor 12is preferably a centrifugal compressor, however any other suitable typeof compressor can be used. The compressor 12 is driven by the steamturbine 14, which can drive the compressor 12 at either a single speedor at variable speeds. Preferably, the steam turbine 14 is a multistage,variable speed turbine that is capable of operating the compressor 12 ata speed that more closely optimizes the efficiency of the chiller system10. More preferably, the steam turbine 14 is capable of driving thecompressor 12 at speeds in a range of about 3200 rpm to about 4500 rpm.The steam turbine 14 is preferably supplied with dry saturated steamfrom one or both of a high pressure steam source 301 and a low pressuresteam source 302. The high pressure steam source 301 can provide steamwithin a range of about 90 to about 200 psi and the low pressure steamsource 302 can provide steam within a range of about 10 to about 20 psi.

A high pressure inlet steam supply valve 68 can control the flow ofsteam from the high pressure steam source 301. Similarly, a low pressureinlet steam supply valve 69 can control the flow of steam from the lowpressure steam source 302. The flow of steam from the high pressuresteam source 301 and/or the low pressure steam source 302 to steamturbine 14 can be further modulated by a governor 48 to vary the speedof the steam turbine 14, and therefore vary the speed of compressor 12to adjust the capacity of the compressor 12 by providing a greater (orlarger) or lesser (or smaller) amount of refrigerant volumetric flowthrough the compressor 12. In another embodiment, the steam turbine 14can drive the compressor 12 at only a single speed and other techniquesare needed to adjust the capacity of the compressor 12, e.g., the use ofpre-rotation vanes 80 and/or a hot gas bypass valve 84 (which devicescan also be used with a variable speed compressor).

The refrigerant vapor delivered by the compressor 12 to the refrigerantcondenser 16 enters into a heat exchange relationship with a fluid,e.g., air or water, and undergoes a phase change to a refrigerant liquidas a result of the heat exchange relationship with the fluid. In apreferred embodiment, the refrigerant vapor delivered to the refrigerantcondenser 16 enters into a heat exchange relationship with a fluid,preferably water, flowing through a heat-exchanger coil connected to acooling tower. The refrigerant vapor in the refrigerant condenser 16undergoes a phase change to a refrigerant liquid as a result of the heatexchange relationship with the fluid in the heat-exchanger coil. Thecondensed liquid refrigerant from refrigerant condenser 16 flows throughan expansion device 22 to the evaporator 18.

The evaporator 18 can include a heat-exchanger coil having a supply line38 and a return line 40 connected to a cooling load. A secondary liquid,e.g., water, ethylene or propylene glycol mixture, calcium chloridebrine or sodium chloride brine, travels into the evaporator 18 via thereturn line 40 and exits the evaporator 18 via the supply line 38. Theliquid refrigerant in the evaporator 18 enters into a heat exchangerelationship with the secondary liquid to lower the temperature of thesecondary liquid. The refrigerant liquid in the evaporator 18 undergoesa phase change to a refrigerant vapor as a result of the heat exchangerelationship with the secondary liquid. The vapor refrigerant in theevaporator 18 exits the evaporator 18 and returns to the compressor 12by a suction line to complete the cycle. It is to be understood that anysuitable configuration of refrigerant condenser 16 and evaporator 18 canbe used in the chiller system 10, provided that the appropriate phasechange of the refrigerant in the refrigerant condenser 16 and evaporator18 is obtained.

At the input or inlet to the compressor 12 from the evaporator 18, thereare one or more pre-rotation vanes (PRV) or inlet guide vanes 80 thatcontrol the flow of refrigerant to the compressor 12, and therebycontrol the capacity of the compressor 12. Pre-rotation vanes 80 arepositionable to any position between a substantially open position,wherein refrigerant flow is essentially unimpeded into the compressor12, and a substantially closed position, wherein refrigerant flow intothe compressor 12 is restricted. It is to be understood that in theclosed position, pre-rotation vanes 80 may not completely stop the flowof refrigerant into the compressor 12. An actuator is used to open thepre-rotation vanes 80 to increase the amount of refrigerant to thecompressor 12 and thereby increase the cooling capacity of the system10. Similarly, the actuator is used to close the pre-rotation vanes 80to decrease the amount of refrigerant to the compressor 12 and therebydecrease the cooling capacity of the system 10. The actuator for thepre-rotation vanes 80 can open and close the pre-rotation vanes 80 ineither a continuous manner or in a stepped or incremental manner.

The chiller system 10 can also include a hot gas bypass connection andcorresponding valve 84 that connects the high pressure side and the lowpressure side of the chiller system 10. In the embodiment illustrated inFIG. 3, the hot gas bypass connection and the hot gas bypass valve 84connect the refrigerant condenser 16 and the evaporator 18 and bypassthe expansion device 22. In another embodiment, the hot gas bypassconnection and hot gas bypass valve 84 can connect the compressorsuction line and the compressor discharge line. The hot gas bypass valve84 is preferably used as a recirculation line for compressor 12 torecirculate refrigerant gas from the discharge of compressor 12, viarefrigerant condenser 16, to the suction of compressor 12, via theevaporator 18. The hot gas bypass valve 84 can be adjusted to anyposition between a substantially open position, wherein refrigerant flowis essentially unimpeded, and a substantially closed position, whereinrefrigerant flow is restricted. The hot gas bypass valve 84 can beopened and closed in either a continuous manner or in a stepped orincremental manner. The opening of the hot gas bypass valve 84 canincrease the amount of refrigerant gas supplied to the compressorsuction to prevent surge conditions from occurring in compressor 12.

With regard to the steam turbine system, the high pressure steam source301 and the low pressure steam source 302 provide steam to the steamturbine 14. The steam from the high pressure steam source 301 and thelow pressure steam source 302 preferably enters a corresponding moistureseparator (not shown) for each steam source. In the moisture separator,moisture-laden steam from the steam source enters and is deflected in acentrifugally downward motion. The entrained moisture in the steam isseparated out by a reduction in the velocity of the steam flow.Separated moisture then falls through a moisture outlet and drysaturated steam flows upward and exits through a steam outlet where itflows toward a corresponding inlet steam supply valve.

The controller 90 automatically positions the high pressure inlet steamsupply valve 68 and the low pressure inlet steam supply valve 69 tocontrol the amount of steam that flows toward a governor 48 during theoperation of the steam turbine 14. The governor 48 is located in thesteam supply line to regulate steam flow and is preferably locatedadjacent a steam inlet of steam turbine 14. The governor or governorvalve 48 can be opened or closed in a continuous manner or in a steppedor incremental manner. Steam turbine 14 includes a steam inlet toreceive the steam from the high pressure steam source 301 and/or the lowpressure steam source 302. The steam from the high pressure steam source301 and/or the low pressure steam source 302 flows through the steaminlet and turns a rotatable turbine portion of the steam turbine 14 toextract the energy therefrom to turn a coupler 66 that interconnects theshafts (not shown) of the steam turbine 14 and compressor 12. Afterrotating the turbine portion of the steam turbine 14, the steam thenexits the steam turbine 14 through a steam exhaust.

In a preferred embodiment, the coupler 66 provides for a directrotational connection between the steam turbine 14 and the compressor12. In alternate embodiments, the coupler 66 can include one or moregearing arrangements (or other similar arrangements) to increase ordecrease the relative rotational speeds between the steam turbine 14 andthe compressor 12. In addition, one or both of the steam turbine 14 andcompressor 12 can also include an internal gearing arrangement connectedto the coupler 66 to adjust the relative rotational speeds of the steamturbine 14 or compressor 12.

In addition, a turbine steam ring drain solenoid valve 63 is provided toautomatically remove any condensate from the steam turbine 14 during theslow roll warm up of the steam turbine 14. A gland seal steam supplysolenoid valve 67 is provided to automatically admit steam to the glandseal supply pressure regulating valve during a slow roll. A steamcondenser vacuum pump 65 evacuates the steam condenser and turbineexhaust to a desired vacuum that is required for the steam turbine 14 toproduce the power required by the compressor 12.

The exhausted steam from the steam turbine 14 flows to the steamcondenser 20. Within the steam condenser 20, the steam/condensate flowfrom the steam turbine 14 enters into a heat exchange relationship withcooling water flowing through the steam condenser 20 to cool the steam.Steam condenser 20 includes a hotwell 44 connected to a condensaterecirculation system 46. Condensate recirculation system 46 includes acondensate outlet in the hotwell 44 that can provide or transfercondensate from the hotwell 44 to a condensate pump 62. From thecondensate pump 62, the condensate is selectively provided to acondensate recirculation inlet of the steam condenser 20 and/or to acondensate return inlet of the high pressure steam source 301 and/or thelow pressure steam source 302. In this manner, the condensaterecirculation system 46 can maintain a preselected flow of condensatethrough the steam condenser 20 and return condensate to the highpressure steam source 301 and/or the low pressure steam source 302 forfurther generation of steam.

As discussed above, cooling water from a cooling tower or other source,is preferably routed to the refrigerant condenser 16 by a cooling watersupply line 70. The cooling water is circulated in the refrigerantcondenser 16 to absorb heat from the refrigerant gas. The cooling waterthen exits the refrigerant condenser 16 and is routed or provided to thesteam condenser 20. The cooling water is circulated in the steamcondenser 20 to further absorb heat from the steam exhausted from thesteam turbine 14. The cooling water flowing from the steam condenser 20is directed to the cooling tower by a cooling water return line 76 toreduce the temperature of the cooling water, which then may be returnedto the refrigerant condenser 16 to repeat the cycle.

Typically, the steam condenser 20 operates at a greater temperature thanthe refrigerant condenser 16. By routing the cooling water through therefrigerant condenser 16 and then the steam condenser 20, in a series orserial arrangement, the low temperature cooling water can absorb heatwithin the refrigerant condenser 16 then be transferred to the steamcondenser 20 to absorb additional heat. In a preferred embodiment, thisability to use the cooling water to cool both the refrigerant condenser16 and the steam condenser 20 can be accomplished by selecting theappropriate refrigerant condenser 16 and steam condenser 20. Therefrigerant condenser 16 is selected such that the outlet cooling watertemperature from the refrigerant condenser 16 is lower than the maximumacceptable inlet cooling water temperature for the steam condenser 20.This series or serial flowpath for condenser (refrigerant and steam)cooling water within the chiller system 10 can reduce the need formultiple supplies of cooling water, and can reduce the total amount ofcooling water required for the chiller system. However, it is to beunderstood that the steam condenser 20 and the refrigerant condenser 16can have separate cooling water systems and connections to the coolingtower.

As illustrated in FIG. 4, the control panel 90 includes analog todigital (A/D) and digital to analog (D/A) converters, a microprocessor96, a non-volatile memory or other memory device 92, and an interfaceboard 98 to communicate with various sensors and control devices ofchiller system 10. In addition, the control panel 90 can be connected toor incorporate a user interface 94 that permits an operator to interactwith the control panel 90. The operator can select and enter commandsfor the control panel 90 through the user interface 94. In addition, theuser interface 94 can display messages and information from the controlpanel 90 regarding the operational status of the chiller system 10 forthe operator. The user interface 94 can be located locally to thecontrol panel 90, such as being mounted on the chiller system 10 or thecontrol panel 90, or alternatively, the user interface 94 can be locatedremotely from the control panel 90, such as being located in a separatecontrol room apart from the chiller system 10.

Microprocessor 96 executes or uses a single or central control algorithmor control system to control the chiller system 10 including thecompressor 12, the steam turbine 14, the steam condenser 20 and theother components of the chiller system 10. In one embodiment, thecontrol system can be a computer program or software having a series ofinstructions executable by the microprocessor 96. In another embodiment,the control system may be implemented and executed using digital and/oranalog hardware by those skilled in the art. In still anotherembodiment, the control panel 90 may incorporate multiple controllers,each performing a discrete function, with a central controller thatdetermines the outputs of control panel 90. If hardware is used toexecute the control algorithm, the corresponding configuration of thecontrol panel 90 can be changed to incorporate the necessary componentsand to remove any components that may no longer be required.

The control panel 90 of the chiller system 10 can receive many differentsensor inputs from the components of the chiller system 10. Someexamples of sensor inputs to the control panel 90 are provided below,but it is to be understood that the control panel 90 can receive anydesired or suitable sensor input from a component of the chiller system10. Some inputs to the control panel 90 relating to the compressor 12can be from a compressor discharge temperature sensor, a compressor oiltemperature sensor, a compressor oil supply pressure sensor and apre-rotation vane position sensor. Some inputs to the control panel 90relating to the steam turbine 14 can be from a turbine shaft end bearingtemperature sensor, a turbine governor end bearing temperature sensor, aturbine inlet steam temperature sensor, a turbine inlet steam pressuresensor, a turbine first stage steam pressure sensor, a turbine exhaustpressure sensor, a turbine speed sensor, and a turbine trip valve statussensor.

Some inputs to the control panel 90 relating to the steam condenser 20can be from a hotwell condensate level sensor, a hotwell high levelstatus sensor, and a hotwell low level status sensor. Some inputs to thecontrol panel 90 relating to the refrigerant condenser 16 can be from anentering refrigerant condenser water temperature sensor, a leavingcondenser water temperature sensor, a refrigerant liquid temperaturesensor, a refrigerant condenser pressure sensor, a subcooler refrigerantliquid level sensor, and a refrigerant condenser water flow sensor. Someinputs to the control panel 90 relating to the evaporator 18 can be froma leaving chilled liquid temperature sensor, a return chilled liquidtemperature sensor, an evaporator refrigerant vapor pressure sensor, arefrigerant liquid temperature sensor, and a chilled water flow sensor.In addition, other inputs to the controller 90 include a HVAC&R demandinput from a thermostat or other similar temperature control system.

Furthermore, the control panel 90 of the chiller system 10 can provideor generate many different control signals for the components of thechiller system 10. Some examples of control signals from the controlpanel 90 are provided below, but it is to be understood that the controlpanel 90 can provide any desired or suitable control signal for acomponent of the chiller system 10. Some control signals from thecontrol panel 90 can include a turbine shutdown control signal, acompressor oil heater control signal, a variable speed oil pump controlsignal, a turbine governor valve control signal, a hotwell level controlsignal, a hot gas bypass valve control signal, a subcooler refrigerantliquid level control signal, a pre-rotation vane position controlsignal, and steam inlet valve control signals. In addition, the controlpanel 90 can send a turbine shutdown signal when either the technicianhas input a shutdown command into the user interface 94, or when adeviation is detected from a preselected parameter recorded in thememory device 92.

The central control algorithm executed by the microprocessor 96 on thecontrol panel 90 preferably includes a startup control program oralgorithm to control the startup of the steam turbine 14 and compressor12. The startup control program and the integration of controls in thecontrol panel 90 provides for additional protections for individualcomponents in the event of an off-design operating condition in thesteam turbine 14 or the chiller system 10. The startup control programprovides automatic shutdown logic and protective functions to protectthe chiller system 10 during operation. These protective functionsinclude a pre-lubrication for the compressor 12 and steam turbine 14 toensure that adequate lubrication is provided prior to rotating thecompressor 12 and steam turbine 14. These protective systems alsoinclude a time sharing for redundant equipment such as hotwell pumps andvacuum pumps, wherein equipment are selectively operated in an alternatefashion to provide greater long term reliability.

In addition, the central control algorithm can maintain selectedparameters of the chiller system 10 within preselected ranges. Theseparameters include turbine speed, chilled liquid outlet temperature,turbine power output, and anti-surge limits for minimum compressor speedand compressor pre-rotation vane position. The central control programemploys continuous feedback from sensors monitoring various operationalparameters described herein to continuously monitor and change the speedof turbine 14 and compressor 12 in response to changes in system coolingloads.

The central control algorithm also includes other algorithms and/orsoftware that provide the control panel 90 with a monitoring function ofvarious operational parameters for the chiller system 10 during bothstartup and routine operation of the chiller system 10. Undesirableoperational parameters, such as low turbine speed, low turbine oilpressure, or low compressor oil pressure, can be programmed into thecontrol panel 90 with a logic function to shutdown the chiller system 10in the event that undesired, or beyond system design, parameters aredetected. Additionally, the central control algorithm has preselectedlimits for many of the operational parameters of the chiller system 10and can prevent a technician from manually operating the chiller system10 outside of these limits.

In one embodiment of the present invention, the central controlalgorithm incorporates a governor control system either as a separateprogram or as a subprogram of the central control algorithm. Thegovernor control system is used to control the positions of the highpressure inlet steam supply valve 68, the low pressure inlet steamsupply valve 69 and the governor valve 48 during the start-up, slow rolland shut down of the compressor 14. The governor control system cangenerate the appropriate control signals for the valves in response tosystem parameters.

FIG. 5 illustrates an embodiment of an automatic start-up process forthe control program of the present invention. The start-up processbrings the chiller system 10 out of a shutdown state and starts theturbine 14 slow rolling or idling. The start-up process begins at step502 with the execution of an initiation sequence for the chiller system10. In step 502, the initiation sequence can include, among other steps,the resetting of the controller logic to clear any safeties that mayhave been set in the controller logic and the checking of all systems inthe chiller system 10 to ensure readiness for operation. In step 504,the operator is able to select whether the start-up process is to becompleted with low pressure steam or with high pressure steam. In apreferred embodiment, if a selection of either low pressure steam orhigh pressure steam is not made within a predetermined steam selectiontime period, e.g., about 1 minute, the start-up process uses lowpressure steam.

Condenser water flow to the chiller system 10 (particularly the steamcondenser 20) is started in step 506. The condenser water flow ispreferably set to a predetermined start-up condenser water flow rate,e.g., about 3000 gpm (gallons per minute). Once the condenser water flowreaches a predetermined minimum start-up condenser water flow rate,e.g., about 2000 gpm, for a predetermined minimum start-up condenserwater flow time period, e.g., about 30 seconds, the oil pumps for thechiller system 10 are started and pre-lube and slow roll warm-upsequences are initiated in step 508. In addition, in step 508, the steamcondenser hotwell pump and vacuum pump can be started after apredetermined pre-lube time period, e.g., about 30 seconds.

In step 510, the governor valve 48 is opened by setting a turbine speedsetpoint to a predetermined slow roll speed, e.g., about 500 rpm(revolutions per minute) at a predetermined slow roll ramp rate, e.g.,about 50 rpm/sec. Once the governor valve 48 has reached a predeterminedslow roll governor valve position, e.g., about 5% open, the highpressure inlet steam supply valve 68 is opened and the turbine 14 canbegin to slow roll in step 512. In step 514, the speed of the turbine 14is checked to see if it is greater than a predetermined minimum slowroll speed, e.g., about 200 rpm. If the turbine speed is not greaterthan the predetermined minimum slow roll speed in step 514, then thegovernor valve 48 and the high pressure inlet steam supply valve 68 arecontinued to be opened. However, if the turbine speed is greater thanthe predetermined minimum slow roll speed in step 514, then the turbine14 is considered to be “slow rolling” and the compressor oil coolingsystem is started in step 516.

In one embodiment of the present invention, the compressor oil coolingsystem is controlled based on the temperature of the thrust bearing oilin order to prevent over cooling of the compressor oil during theextended slow roll and idling periods. The compressor oil cooling systemcontrols the activation and deactivation of both an oil heater and acooling water supply that supplies cooling water to the compressor oilcooler. The cooling liquid supply is started when the thrust bearing oiltemperature is greater than a predetermined maximum cooling supplytemperature, e.g., about 155° F., and stopped when the thrust bearingoil temperature decreases below a predetermined minimum cooling supplytemperature, e.g., about 140° F. The oil heater is started if the oiltemperature is less than a predetermined minimum oil heater temperature,e.g., about 130° F., and stopped if the thrust bearing oil temperatureincreases above a predetermined maximum oil heater temperature, e.g.,about 150° F.

Finally, in step 518, the turbine 14 is ramped up to the predeterminedslow roll speed. If the low pressure steam option was selected in step504, then the turbine 14 is to be slow rolled with low pressure steam.In this case, the slow roll of the turbine 14 is transitioned to lowpressure steam after the turbine 14 has been slow rolling for apredetermined minimum slow roll speed time period, e.g., about 4minutes. To make the transition, the low pressure inlet steam supplyvalve 69 is opened to a predetermined slow roll LP inlet steam supplyvalve position, e.g., about 10% open. When the low pressure inlet steamsupply valve 69 starts to open, the high pressure inlet steam supplyvalve 68 is closed. The governor control system then controls the lowpressure inlet steam supply valve 69 to maintain the speed at thepredetermined slow roll speed. If the high pressure steam option wasselected in step 504, then the turbine 14 is to be slow rolled with highpressure steam. The high pressure steam option is preferably selectedwhen the operator requires the turbine 14 to ramp the chiller up torated speed as soon as available.

Once the turbine 14 has reached the predetermined slow roll speed instep 518, the turbine 14 begins a predetermined slow roll warm up timeperiod, e.g., about 26 minutes, to ensure all condensate is blown out ofthe inlet piping, the casing is uniformly heated, and the turbine shaftis not bowed due to sitting idle. After the predetermined slow roll warmup time period, if the turbine exhaust pressure is at or below apredetermined slow roll vacuum, e.g., about 24 in. Hg vac., the userinterface 94 displays “TURBINE IDLING”. Otherwise, if the turbineexhaust pressure is not below the predetermined slow roll vacuum, theuser interface 94 displays “TURBINE IDLING—INSUFF VACUUM”. This warningcould indicate a problem with the steam ejectors or an excessive leakrequiring investigation by the operator.

Once the turbine is idling properly after the predetermined warm up timeperiod, if the operator has selected low pressure steam and an idlingmode of operation for the turbine 14, the turbine 14 continues to slowroll with low pressure steam. The governor control system continues tocontrol the low pressure inlet steam supply valve 69 to maintain theturbine speed at the predetermined slow roll speed. The turbine 14 isthen ready to ramp up to rated speed as described in FIGS. 6 and 7 inresponse to the operator's command. However, if the operator hasselected high pressure steam and a rated speed mode of operation for theturbine 14, the turbine 14 can proceed directly to ramping up to ratedspeed as described in FIGS. 6 and 7. In one embodiment of the presentinvention, the governor control system can use a new set of tuningparameters when the vacuum level is below a preselected level, whichdepends on the steam supply pressure, to prevent instability.

FIGS. 6 and 7 illustrate an embodiment of the ramp-up to rated speedprocess for the control program of the present invention. The ramp-up torated speed process transitions the turbine 14 from a slow rolling oridling speed to an operational speed sufficient to drive the compressor12 of the chiller system 10. The process begins at step 602 to determineif the turbine 14 is slow rolling using low pressure steam. If theturbine 14 is slow rolling using low pressure steam, the controlproceeds to step 604. Otherwise, the turbine is slow rolling with highpressure steam and the control proceeds to step 608. In step 604, theturbine 14 is transitioned from low pressure steam to high pressuresteam. To make the transition, the high pressure inlet steam supplyvalve 68 is opened to a predetermined slow roll HP inlet steam supplyvalve position, e.g., about 6% open. When the high pressure inlet steamsupply valve 68 starts to open, the low pressure inlet steam supplyvalve 69 is closed. The governor control system then controls the highpressure inlet steam supply valve 68 to maintain the turbine speed atthe predetermined slow roll speed.

In step 606, the turbine 14 is slow rolled or idled using high pressuresteam for a predetermined HP warm up time period, e.g., about 15minutes. The high pressure steam slow roll is required to ensure thatall turbine components are uniformly heated to the higher temperaturebefore ramping to rated speed. Once the predetermined HP warm up timeperiod expires, the turbine 14 is ready to begin the process of rampingup to rated speed and the control proceed to step 608. At step 608, thecondenser water flow is then increased to a predetermined ramp-upcondenser water flow rate, e.g., about 9400 gpm (gallons per minute).Once the condenser water flow reaches the predetermined ramp-upcondenser water flow rate, the evaporator water flow is then set to apredetermined ramp-up evaporator water flow rate, e.g., about 3750 gpm(gallons per minute). Once the condenser and evaporator water flow ratesare stable, the control proceeds to step 610.

In step 610, the turbine speed setpoint is set to a predeterminedminimum turbine speed, e.g., about 2000 rpm, at a predetermined minimumturbine speed ramp rate, e.g., about 50 rpm/sec. As a result ofadjusting the turbine speed setpoint, the governor valve 48 and the highpressure inlet steam supply valve 68 are both further opened by thegovernor control system. In step 612, the turbine speed is checked todetermine if it is greater than a predetermined ramp up turbine speed,e.g., about 1000 rpm. If the turbine speed is less than thepredetermined ramp up turbine speed, the turbine 14 is continued to beaccelerated in accordance with step 610. However, if the turbine speedis greater than the predetermined ramp up turbine speed, then thepre-rotation vanes 80 are opened to a predetermined PRV ramp upposition, e.g., about 18% open, in step 614.

In step 616, the turbine speed is checked to determine if it is greaterthan the predetermined minimum turbine speed. If the turbine speed isless than the predetermined minimum turbine speed, the turbine 14 iscontinued to be accelerated in accordance with step 610. However, if theturbine speed is greater than the predetermined minimum turbine speed,then the turbine speed setpoint is set to a predetermined critical speedrange turbine speed, e.g., about 2500 rpm, at a predetermined criticalspeed range turbine speed ramp rate, e.g., about 100 rpm/sec, in step618. In step 620, the turbine speed is checked to determine if it isgreater than the predetermined critical speed range turbine speed. Ifthe turbine speed is less than the predetermined critical speed rangeturbine speed, the turbine 14 is continued to be accelerated inaccordance with step 618. However, if the turbine speed is greater thanthe predetermined critical speed range turbine speed, then the turbinespeed setpoint is set to a predetermined rated turbine speed, e.g.,about 3000 rpm, at a predetermined rated turbine speed ramp rate, e.g.,about 50 rpm/sec, and the turbine steam ring drain valve 63 is closed instep 622.

In step 624, the turbine speed is checked to determine if it is greaterthan a predetermined operational turbine speed, e.g., about 2700 rpm. Ifthe turbine speed is less than the predetermined operational turbinespeed, the turbine 14 is continued to be accelerated in accordance withstep 622. However, if the turbine speed is greater than thepredetermined operational turbine speed, then the elapsed time theturbine has been operating a speed greater than the predeterminedoperational turbine speed is compared to a predetermined operationalturbine speed first time period, e.g., 15 seconds, in step 626. If theelapsed time is less than the predetermined operational turbine speedfirst time period, the turbine 14 is continued to be accelerated inaccordance with step 622. However, if the elapsed time is greater thanthe predetermined operational turbine speed first time period, then theturbine 14 is considered to have reached its minimum rated speed and theuser interface 94 displays “System Running” in step 628.

In step 630, the elapsed time the turbine has been operating at a speedgreater than the predetermined operational turbine speed is compared toa predetermined operational turbine speed second time period, e.g., 25seconds. If the elapsed time is less than the predetermined operationalturbine speed second time period, the turbine 14 is continued to beoperated in accordance with step 622. However, if the elapsed time isgreater than the predetermined operational turbine speed second timeperiod, then the capacity control logic is started in step 632. When thecapacity control logic is started in step 632, the hot gas bypass valve84 begins to close and the compressor pre-rotation vanes 80 begin toopen. The high pressure inlet steam supply valve 68 is ramped slowly toa fully open position, i.e., 100%, at a predetermined HP inlet steamsupply valve opening rate, e.g., 1%/second. If the turbine 14 attemptsto speed up with the increased steam flow, the capacity control systemcloses the governor valve 48 and maintains the turbine speed at the setpoint dictated by the capacity/anti-surge controls.

FIGS. 8 and 9 illustrate an embodiment of the return to idling speedprocess for the control program of the present invention. The return toidling speed process transitions the turbine 14 from an operational orrated speed sufficient to drive the compressor 12 of the chiller system10 to a slow rolling or idling speed. The process begins at step 802with the initiation of a predetermined controlled stop time period,e.g., 30 minutes. Next, during the predetermined controlled stop timeperiod, the high pressure inlet steam supply valve 68 is closed at apredetermined HP inlet steam supply valve closing rate, e.g.,−2%/second, at step 804. The high pressure inlet steam supply valve 68is continued to be closed during the predetermined controlled stop timeperiod until the position of the high pressure inlet steam supply valve68 permits it to be controlled by the governor control system, i.e., theposition of the high pressure inlet steam supply valve 68 is more closedthan or at the same position as the determined position by the governorcontrol system for the high pressure inlet steam supply valve 68. Oncethe high pressure inlet steam supply valve 68 is under the control ofthe governor control system, a normal unloading cycle is initiated atstep 808. In the normal unloading cycle, the leaving chilled watertemperature setpoint is slowly increased at a preselected rate, e.g.,0.1° F./5 seconds. The turbine speed decreases to a calculated minimumanti-surge RPM, then the compressor pre-rotation vanes 80 are closed tothe calculated minimum anti-surge % opening, and finally the hot gasbypass valve 84 is opened.

The chiller system 10 continues to slowly unload until the hot gasbypass valve 84 is more than a predetermined controlled stop hot gasvalve position, e.g., 20% open, or the predetermined controlled stoptime period has expired. Once the hot gas bypass valve 84 is more openthan the predetermined controlled stop hot gas valve position or thepredetermined controlled stop time period has expired, the high pressureinlet steam supply valve 68 is closed in step 812. In addition, theexhaust of the turbine 14 is opened to atmospheric pressure to slow theturbine 14 down through the critical speed range as rapidly as possible.In step 814, the turbine speed is checked to determine if it is lessthan a predetermined controlled stop first turbine speed, e.g., about2400 rpm. If the turbine speed is greater than the predeterminedcontrolled stop first turbine speed, the turbine 14 is continued to bedecelerated in accordance with step 808. However, if the turbine speedis less than the predetermined controlled stop first turbine speed, thenthe leaving chilled water temperature setpoint is set to track theleaving chilled water temperature and the compressor pre-rotation vanes80 are set to a predetermined PRV controlled stop position, e.g., about18% open, in step 816.

Next, in step 818, the high pressure inlet steam supply valve 68 ischecked to see if it is fully closed and the turbine speed is checked todetermine if it is less than a predetermined controlled stop secondturbine speed, e.g., about 1800 rpm. If both conditions are satisfied instep 818, the control proceeds to step 820. Otherwise, the turbine speedis decelerated in accordance with step 816. In step 820, the governorcontrol system begins to control the speed of the turbine 14 with thelow pressure inlet steam supply valve 69. In addition, the turbine speedsetpoint is set to the predetermined slow roll speed, e.g., about 500rpm, at a predetermined controlled stop ramp rate, e.g., about −50rpm/sec.

Once the turbine speed is less than the predetermined controlled stopsecond turbine speed, the hot gas bypass valve 84 is fully opened instep 824. In addition, the vacuum pump of the turbine 14 is started tore-establish a vacuum in the turbine 14. In step 826, the turbine speedis checked to determine if it is less than a predetermined ramp downturbine speed, e.g., about 1000 rpm. If the turbine speed is greaterthan the predetermined ramp down turbine speed, the turbine 14 iscontinued to be decelerated in accordance with step 820. However, if theturbine speed is less than the predetermined ramp down turbine speed,then slow roll or idling mode operation is initiated in step 828. Theinitiation of the slow roll mode of operation includes the shut down ofthe evaporator water flow and the setting of the condenser water flow tothe predetermined start-up condenser water flow rate. Furthermore, thepre-rotation vanes 84 are fully closed and the user interface displaysthe message “Turbine Idling”. The turbine 14 is then idled at thepredetermined slow roll speed by controlling the low pressure inletsteam supply valve 69 until the operator decides to either shut down thechiller system 10 or ramp up the turbine speed to an operational speedas described above with respect to FIGS. 6 and 7.

In one embodiment of the present invention, if a complete shutdown ofthe chiller system is required, e.g., in an emergency situation, theabove process for returning to idling speed is followed except that theturbine speed is not maintained at the predetermined slow roll speed,but is permitted to coast down to zero. Once the turbine speed reaches apredetermined shut down turbine speed, e.g., 200 rpm, the turbine steamring drain valve 63 is opened, the condenser water flow is stopped andthe hotwell pump(s) are stopped. Finally, the compressor and turbineauxiliary oil pumps are operated for predetermined time periods afterthe stop of the turbine 14 to prevent damage to the turbine 14 andcompressor 12.

While the invention has been described with reference to a preferredembodiment, it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted forelements thereof without departing from the scope of the invention. Inaddition, many modifications may be made to adapt a particular situationor material to the teachings of the invention without departing from theessential scope thereof. Therefore, it is intended that the inventionnot be limited to the particular embodiment disclosed as the best modecontemplated for carrying out this invention, but that the inventionwill include all embodiments falling within the scope of the appendedclaims.

1. A method of starting a steam turbine driven chiller system having ahigh pressure steam supply and a low pressure steam supply, the methodcomprising the steps of: executing a starting sequence for the steamturbine; initiating a slow roll of the steam turbine using the highpressure steam supply; transitioning from the high pressure steam supplyto the low pressure steam supply; and slow rolling the steam turbine ata predetermined slow roll speed using the low pressure steam supply. 2.The method of claim 1 wherein the step of initiating a slow roll of thesteam turbine includes opening a governor valve of the steam turbine. 3.The method of claim 2 wherein the step of initiating a slow roll of thesteam turbine further includes opening a high pressure steam inletsupply valve in response to the governor valve being opened to apredetermined position.
 4. The method of claim 1 wherein the step oftransitioning from the high pressure steam supply to the low pressuresteam supply includes operating the steam turbine using the highpressure steam supply for a predetermined time period.
 5. The method ofclaim 4 wherein the step of transitioning from the high pressure steamsupply to the low pressure steam supply further includes the steps of:opening a low pressure inlet steam supply valve in response to thepredetermined time period expiring; and closing a high pressure inletsteam valve in response to the low pressure steam inlet supply valvebeginning to open.
 6. The method of claim 5 wherein the step of slowrolling the steam turbine at a predetermined slow roll speed using thelow pressure steam supply includes positioning the low pressure inletsteam supply valve to maintain the predetermined slow roll speed.
 7. Themethod of claim 1 wherein the step of slow rolling the steam turbine ata predetermined slow roll speed using the low pressure steam supplyincludes slow rolling the steam turbine at a predetermined slow rollspeed for a predetermined time period.
 8. The method of claim 1 whereinthe predetermined slow roll speed is about 500 rpm.
 9. The method ofclaim 1 further comprising the step of operating the steam turbine at apredetermined operational speed using the high pressure steam supply.10. The method of claim 9 wherein the step of operating the steamturbine at a predetermined operational speed includes transitioning fromthe low pressure steam supply to the high pressure steam supply.
 11. Themethod of claim 10 wherein the step of transitioning from the lowpressure steam supply to the high pressure steam supply includes thesteps of: opening a high pressure inlet steam supply valve; closing alow pressure inlet steam valve in response to the high pressure steaminlet supply valve beginning to open; and slow rolling the steam turbineat the predetermined slow roll speed using the high pressure steamsupply for a predetermined time period.
 12. A method of initiating anidling mode in a steam turbine driven chiller system having a highpressure steam supply and a low pressure steam supply, the methodcomprising the steps of: executing a transition sequence for the steamturbine, the steam turbine operating at a rated speed using the highpressure steam supply prior to the transition sequence; initiating anunload cycle for the chiller system; transitioning from the highpressure steam supply to the low pressure steam supply; and slow rollingthe steam turbine at a predetermined idling speed using the low pressuresteam supply, the predetermined idling speed being less than the ratedspeed.
 13. The method of claim 12 wherein the step of executing atransition sequence for the steam turbine includes initiating apredetermined controlled stop time period.
 14. The method of claim 13wherein the step of executing a transition sequence for the steamturbine further includes the steps of: closing a high pressure steaminlet valve from a fully open position during the predeterminedcontrolled stop time period; and controlling the high pressure steaminlet valve with a control system in response to the high pressure steaminlet valve being closed to a predetermined position.
 15. The method ofclaim 14 wherein the step of initiating an unload cycle for the chillersystem occurs in response to the control system controlling the highpressure steam inlet valve.
 16. The method of claim 13 wherein the stepof initiating an unload cycle for the chiller system includes the stepsof: increasing a leaving chilled water setpoint temperature for anevaporator of the chiller system; decreasing the speed of the steamturbine to a predetermined turbine speed to avoid a surge condition in acompressor of the chiller system; closing pre-rotation vanes of thecompressor; and opening a hot gas bypass valve of the chiller system.17. The method of claim 16 wherein the step of transitioning from thehigh pressure steam supply to the low pressure steam supply occurs inresponse to one of the expiration of the predetermined controlled stoptime period or the hot gas bypass valve being open more than apredetermined hot gas bypass valve position.
 18. The method of claim 12wherein the step of transitioning from the high pressure steam supply tothe low pressure steam further includes the steps of: closing a highpressure inlet steam valve; and controlling turbine speed with a lowpressure steam inlet valve in response to the high pressure inlet steamvalve being closed and the speed of the turbine being less than apredetermined first turbine speed, the predetermined first turbine speedbeing less than the rated speed and greater than the predeterminedidling speed.
 19. The method of claim 18 wherein the step oftransitioning from the high pressure steam supply to the low pressuresteam supply further includes the steps of: opening a hot gas bypassvalve of the chiller system in response to the speed of the turbinebeing less than the predetermined first turbine speed; starting a vacuumpump to establish a vacuum in the steam turbine in response to the speedof the turbine being less than the predetermined first turbine speed;and stopping evaporator water flow in the chiller system, reducingcondenser water flow in the chiller system and closing pre-rotationvanes of a compressor of the chiller system in response to the turbinespeed being less than a predetermined second turbine speed, thepredetermined second turbine speed being less than the predeterminedfirst turbine speed.
 20. The method of claim 12 wherein thepredetermined idling speed is about 500 rpm.
 21. A chiller systemcomprising: a steam system comprising a high pressure steam supply, alow pressure steam supply, a steam turbine and a steam condenserconnected in a steam loop; a refrigerant system comprising a compressor,a refrigerant condenser, and an evaporator connected in a refrigerantloop, wherein the compressor is driven by the steam turbine; a controlpanel to control operation of both the steam system and the refrigerantsystem, the control panel comprising a control system to operate thesteam system in an idling mode using the low pressure steam supply; andwherein idling mode operation results in the steam turbine operating ata predetermined slow roll speed and no substantial output capacity fromthe refrigerant system.
 22. The chiller system of claim 21 wherein thehigh pressure steam supply provides steam within a range of about 90 psito about 200 psi and the low pressure steam supply provides steam withina range of about 10 psi to about 20 psi.
 23. The chiller system of claim21 wherein the control system comprises a control algorithm configuredto automatically control opening and closing of at least one of a highpressure steam inlet valve, a low pressure steam inlet valve, a turbinesteam ring drain valve and a turbine gland seal steam system supplyvalve.
 24. The chiller system of claim 21 wherein the control system isconfigured to transition the steam system from the idling mode to anoperational mode using the high pressure steam supply and to transitionthe steam system from an operational mode using the high pressure steamsupply to the idling mode.
 25. The chiller system of claim 21 whereinthe steam system comprises a governor valve and the control system isconfigured to automatically control opening and closing of the governorvalve, a high pressure steam inlet valve and a low pressure steam inletvalve during the idling mode of operation.